Atmospheric pressure ion sources (API) have become increasingly important as a means for generating ions used in mass analysis. Electrospray or nebulization assisted Electrospray (ES), Atmospheric Pressure Chemical Ionization (APCI) and Inductively Coupled Plasma (ICP) ion sources produce ions from analyte species in a region which is approximately at atmospheric pressure. The ions must then be transported into vacuum for mass analysis. A portion of the ions created in the API source are entrained in the bath gas API source chamber and are swept into vacuum along with a the bath or carrier gas through an orifice into vacuum. Mass spectrometers (MS) generally operate in a vacuum maintained at between 10.sup.-4 to 10.sup.-10 torr depending on the mass analyzer type. The gas phase ions entering vacuum from an API source must be separated from the background carrier gas and transported and focused through a single or multiple staged vacuum system into the mass analyzer. Variations in vacuum system and associated electrostatic lens configurations have emerged in API/MS systems. Where multiple pumping stages have been employed, the electrostatic lens elements have been configured to serve as restricted orifices between vacuum stages as well as providing ion acceleration and focusing of ion into the mass analyzer. Performance tradeoffs may occur where electrostatic lenses must also accommodate restricting the neutral gas transmission from one pumping stage to the next. For example, a skimmer placed between one pumping stage and the next may restrict the neutral gas flow but may also restrict the passage of ions as well due to its relatively small orifice. Two types of Electrostatic elements have been used to transport and focus ions in vacuum, particularly where ions are entering vacuum from atmosphere pressure through a free jet expansion. The first is a static voltage lens and the second is a dynamic field ion guide. The most effective lens configurations used in API/MS systems employ a judicious combination of both elements which have static and dynamic fields applied.
The first electrostatic lens type has a fixed or static DC voltage applied during the time an ion is traversing the lenses' field. FIG. 1 is a diagrammatic representation of a four pumping stage API/MS system with static voltage electrostatic lenses. Gas emerging from the capillary exit 8 into vacuum expands as a supersonic free jet and a portion of the gas passes through the first 10 and second 14 skimmer. Skimmers between pumping stages typically have small orifices to restrict the neutral gas flow into each downstream vacuum stage. DC voltages are applied to the capillary exit, skimmers and other electrostatic lenses 9, 14, 15, 16 and 17 with values set to maximize the ion transmission into the mass spectrometer. Ions entrained in the expanding gas follow trajectories that are driven by a combination of electrostatic and gas dynamic forces. Strong influence from the gas dynamics can extend up to and beyond the second skimmer 13 for the configuration shown in Figure one. The efficiency of ion transmission through a static voltage lens set can be reduced by scattering losses due to collisions between ions and the background gas which occur along the ion trajectory. Ions with different m/z may vary their collisional cross sections and hence experience different numbers of background collisions as they are transported through vacuum. For a given electrostatic lens voltage setting the efficiency of ion transport into the mass spectrometer may vary with m/z or the collisional cross section. Changing the lens voltage values may optimize transmission for a given ion species but the setting may not be optimal for another ion species transmission. For example static lens configurations used in API/MS applications may not transmit lower molecular mass compounds as efficiently as higher molecular mass compounds. The smaller ions may sustain higher transmission losses due to collisional scattering from the background gas than the higher molecular mass compounds. To increase ion transmission efficiency through a static lens stack, the electrostatic energy must be set sufficiently high so that ions can be driven through the background gas. Static voltage lens configurations also tend to focus ions of different energy at different focal points. If the focal point is not located at the mass spectrometer entrance transmission losses can occur. To overcome the mass to charge transmission discrimination effects and ion transport inefficiencies which occur when static voltage lenses are used, multipole dynamic field ion guides have been employed to transport ions through vacuum pumping stages in the vacuum region of API/MS systems. The dynamic electrostatic fields within a multipole ion guide dominate over the background gas scattering collisions and effectively "trap" the ions while they traverse the length of the multipole ion guide.
The use of multipole ion guides has been shown to be an effective means of transporting ions through vacuum. Publications by Olivers et. al. (Anal. Chem, Vol. 59, p. 1230-1232, 1987), Smith et. al. (Anal. Chem. Vol. 60, p.436-441, 1988) and U.S. Pat. No. 4,963,736 (1990) have reported the use of a quadrupole ion guide operated in the AC-only mode to transport ions from an API source into a quadrupole mass analyzer. U.S. Pat. No. 4,963,736 describes the use of a multipole ion guide in either vacuum pumping stage two of a three stage system or in the first pumping stage of a two stage vacuum system. This patent also reports that increasing the background pressure up to 10 millitorr in the vacuum stage where the ion guide was positioned resulted in an increase in ion transmission efficiency and a decrease in ion energy spread of ions transmitted. Ion signal intensity decreased for higher background pressures in the reported quadrupole configuration. A commercially available API/MS instrument manufactured by Sciex, a Canadian company, incorporates a quadrupole ion guide operated in the AC-only mode located before the quadruple mass filter in a single stage vacuum system. Ions and neutral gas flowing into vacuum through an orifice in the API source enter the quadrupole AC-only ion guide. The ions are trapped from expanding in the radial direction by the AC quadrupole fields and are transmitted along the quadrupole ion guide rod length as the neutral gas is pumped away through the rod spacing. Ions exiting the quadrupole ion guide are focused into a quadrupole mass filter located in the same vacuum chamber. Neutral gas is pumped away by a high capacity and relatively expensive cyro pump. Multiple quadrupole ion guides have been used to transport ions from API sources through multiple vacuum pumping stages and into a Fourier-Transform Ion Cyclotron Resonance mass analyzer. Beu et. al. (J. Am. Soc. Mass Spectrom vol. 4. 546-556, 1993) have reported using three quadrupole ion guides operated in the AC-only mode located in three consecutive vacuum pumping stages of a five pumping stage Electrospray Fourier-Transform Ion Cyclotron Resonance (FT-ICR) mass spectrometer instrument. The multiple pumping stages are required to achieve operating pressures in the mass analyzer of less than 2.times.10.sup.-9 torr. Orifices mounted in the partitions between each vacuum pumping stage which restricted neutral gas conductance from one pumping stage to the next were located between consecutive quadrupole ion guides.
Over the past few years as API/MS system design has evolved, higher performance with lower system cost has been achieved by using multiple vacuum stages to remove the background gas from the ions which enter from atmospheric pressure into vacuum. The type of mass analyzer to which an API source is interfaced places its unique demands on the ion transport lens configurations and vacuum requirements in the ion transport region between atmospheric pressure and the mass analyzer. Each mass analyzer type has an acceptable ion energy, ion energy spread and entrance angular divergence which the upstream ion transport lens system must satisfy when delivering ions to the entrance of a mass spectrometer. For example, a quadrupole mass analyzer can accept ions with axial translational energy generally below 40 electron volts whereas a magnetic sector mass spectrometer requires ions with thousands of volts of axial translational energy.
In the present invention, a multipole ion guide is configured to increase the overall sensitivity of an API/MS system while reducing instrument cost and complexity. In one embodiment of the present invention, a multipole ion guide is used to transport ions entering vacuum from an API source to non-dispersion type mass analyzers. A range of ion mass to charge (m/z) can be efficiently transmitted through a multipole ion guide provided the ion guide operating stability region is set to pass those values of m/z. If an ion with a given mass to charge ratio falls within the operating stability region set for a multipole ion guide, the ion will be effectively trapped from drifting to far in the off axis direction but is free to move in the direction of ion guide axis. If the ion m/z falls outside the stability region, it will not have a stable trajectory and will be rejected from the ion guide before it reaches the exit end. Collisions between an ion and the background gas within the multipole assembly can also effect the ion trajectory and the ion kinetic energy as it passes through the multipole ion guide. The background gas, if present at high enough pressure, may serve, through collisions, to damp the motion of ions as they pass through the multipole ion guide, cooling their kinetic and thermal energy. This aids in forming an ion beam which exits the multipole ion guide with reduced energy spread for a given ion species within the beam. The range of m/z which are transmitted through a multipole ion guide for a given background pressure environment can be varied by adjusting the AC frequency and AC and/or a DC voltage which can be applied with alternate polarity to each adjacent rod. The offset potential of the multipole lens, that is the DC voltage applied uniformly to all the rods on which the AC and alternate polarity DC rod potentials are floated and referenced is one variable that can to be used to set the energy of ions transmitted through the multipole ion guide. Multipole ion guides can be configured to efficiently transport ions through a wide range of vacuum pressures. The ability of a multipole ion guide to deliver and ion beam with low energy spread and where the mean energy and m/z range can be adjusted into a mass analyzer can be used to improve the performance of an API/Time-Of-Flight, API/Ion Trap and API/FT-ICR mass spectrometer systems.
Another embodiment of the invention is the incorporation of a multiple vacuum pumping stage multipole ion guide into an API/MS system. A multiple vacuum pumping stage multipole ion guide is a multipole ion guide which begins in one pumping stage and extends contiguously through one or more additional vacuum pumping stages of a multiple pumping stage system. Multipole ion guides which are located in only one vacuum pumping stage of a multiple pumping stage system must deliver the ions exiting the ion guide into an aperture with static voltage applied. If background pressure is high enough to scatter the ions after the multipole ion guide exit or the aperture to the next pumping stage has a smaller diameter than the ion beam cross section, losses in ion transmission can occur. If individual multipole ion guides are located progressively in the first few pumping stages of an API/MS system, ion transmission losses can occur when transferring ions between pumping stages. If fewer pumping stages are used to reduce the ion transmission losses between pumping stages, the total gas flow and hence the total number of ions which can be delivered to vacuum may be compromised. Over 95% ion transmission efficiency can be achieved through multiple vacuum pumping stages using multipole ion guides configured to extend contiguously through two or more vacuum pumping stages. A multiple vacuum stage multipole ion guide must be configured serve as an ion guide with an internal open area small enough to minimize the neutral gas flow from one pumping stage to the next. Xu at. el. (Nuclear Instr. and Methods in Physics Research, Vol. 333, p. 274, 1993) have developed a hexapole lens which extends through two vacuum pumping stages to transport ions formed in a helium discharge source operated in a chamber maintained at 75 to 150 torr of pressure through two vacuum pumping stages into a faraday cup detector. The discharge ion source delivered ions into a two stage vacuum system through an orifice in the end wall of the source chamber. The background pressure in the first vacuum pumping stage was 600 millitorr and the second vacuum stage background pressure was 98 millitorr. Ion transmission efficiencies through the hexapole ion guide beginning in vacuum stage one and extending unbroken into vacuum stage two approached 90% for O.sub.2.sup.+. The helium discharge ion source background pressure in this apparatus was 5 to 10 times below atmospheric pressure and helium was used as the background gas. Different configuration and performance criteria exist for multiple pumping stage multipole ions guides incorporated into an API/MS system than were required for the ion guide application described by Xu and coworkers. Multipole ion guides incorporated into API/MS systems must have the capability of efficiently transmitting ions of various charge states over a wide range of mass to charge. Nitrogen, not helium, is typically used as carrier gas in API sources and the background pressures in API/MS multiple vacuum stage systems are often widely different from the pressures reported in the ion guide apparatus reported by Xu. An added constraint imposed on API/MS systems which was not present in the non API/MS application practiced by Wu et. al. is the ability to fragment molecular ions by Collisional Induced Dissociation (CID) in the gas expansion region in the first two vacuum stages. Valuable structural information can be obtained from CID of molecular ions produced in ES and APCI sources. CID conditions can be set by adjusting relative potentials between static voltage lenses and even the DC offset potentials of multipole ion guides located in the first two vacuum pumping stages of a API source.
In the present invention, multiple pumping stage multipole ion guides are configured to maximize performance of API/MS systems while reducing system vacuum pump cost. Increasing signal sensitivity while lowering vacuum pumping cost is achieved by maximizing the ion transfer efficiency from the API source into the mass analyzer while minimizing the amount of neutral gas transferred. For the multiple pumping stage multipole ion guides which begin in one vacuum pumping stage and extend through one or more subsequent pumping stages, the rod diameter and rod spacing in the multipole ion guide assembly were configured small enough to minimize the transmission of neutral gas through the ion guide into downstream pumping stages. Acceptable vacuum pressure per pumping stage was be achieved with moderate capacity vacuum pumps. The ion guide with a small inner diameter was configured to allow sufficient conduction of neutral gas through the spaces between the rods or poles so the neutral gas was pumped away efficiently in each pumping stage. The small multipole ion guide inner diameter produced an ion beam with a proportionally small cross section. The smaller cross section ion beam focused into the mass analyzer allowed the reduction of the mass analyzer entrance aperture without compromising ion transmission efficiency. Efficient ion transport, better control of ion energy and energy spread and a small beam diameter is achieved by using a multiple vacuum pumping stage multipole ion guide.